Optical Coherence Tomography (OCT) is a technique for high-resolution depth profiling of a sample (biological samples such as tissues, organs, living bodies, or industrial samples such as polymers, thin-films). There are two types of OCT, namely, a time-domain OCT (TD-OCT), and a frequency-domain OCT (FD-OCT). In TD-OCT, the broadband light source is typically a superluminescent diode, which simultaneously emits multiple wavelengths; by scanning the position of a reference mirror, the frequencies of interference components in the reflecting light from the sample are analyzed. In FD-OCT, a swept source type OCT (SS-OCT), which employs a wavelength tunable laser as the broadband source, has become more widely used. In SS-OCT, only one wavelength is present at any one time, and sweeping of the laser wavelength replaces the mechanical scanning of the reference mirror. The signal to noise ratio of SS-OCT is fundamentally better than that of TD-OCT.
For a tunable laser for use in SS-OCT, requirements include: single-mode operation, a wide tuning range, high scan rate of wavelength, and wavelength tuning that is a simple monotonic function of a tuning control signal.
A tunable VCSEL with a MEMS that utilizes two distributed Bragg reflectors (DBR) has been reported. Such a device employs a bottom mirror consisting of a lower DBR composed of multiple alternating layers of AlGalnAs and InP, and an active layer composed of InP-based multiple quantum wells (MQWs) and barriers, which are all grown on a InP substrate, and a MEMS tunable upper DBR. The device has a tuning range of 55 nm at a center wavelength around 1550 nm. This tuning range is not sufficient for a number of applications.
FIG. 1 illustrates such a tunable VCSEL with MEMS, as known in the art. On a InP substrate 1, a n-doped distributed Bragg reflector (DBR) 2 consisting of over 40 pairs (not all shown) of alternating layers of AlGalnAs 2a (lattice-matched to InP) and InP 2b are epitaxially grown, followed by a n-type AlGalnAs cladding layer 3. On the top of the cladding layer 3, an active layer 4 consisting of multiple (six) AlGalnAs quantum wells (“QWs”) 4a and multiple (seven) AlGalnAs barriers 4b are grown, followed by a p-type AlGalnAs cladding layer 5. Above the p-type cladding layer 5, a p++-doped-AlGalnAs/n++-doped-AlGalnAs tunnel junction layer 6 is grown to allow the replacement of a p-type InP layer with a n-doped InP layer since the tunnel junction can convert electrons to holes, which is followed by a n-doped InP layer 7 and a n++-doped GaInAs contact layer 8. VCSEL p-electrode 9 is formed on the top of the contact layer 8 and n-electrode 10 is formed on the substrate 1, to complete the “half VCSEL” structure. On the top of the half VCSEL structure, an independently manufactured upper mirror part is bonded to the half VCSEL structure. The independently manufactured upper mirror part is formed on a “handle” Si-substrate 11 that bonds the two layers together. A SiO2 layer 12 is formed as an insulation layer, followed by a beam support layer of Si 13. A thin membrane 14 is formed by etching the SiO2 layer 12 as a sacrificial layer. An upper dielectric DBR 15 is deposited on one side of the membrane 14, and an antireflection (AR) coating 16 is deposited on the opposite side. A MEMS electrode 17 and Au-bumps 18 are formed to supply the MEMS voltage, which can change the air gap between the contact layer 8 and the upper DBR 15. An electric voltage source 19 is connected with the MEMS electrode 17 and with the p-electrode 9. Therefore, the membrane 14 can be moved by the electrostatic force induced by the electric voltage source 19, thereby changing the cavity length formed between the upper and bottom DBR mirrors, which in turn changes the lasing wavelength. An electric current source 20 is connected for current injection to the half VCSEL part.
Details of a device such as in FIG. 1 are described in T. Yano, H. Saitou, N. Kanbara, R. Noda, S. Tezuka, N. Fujimura, M. Ooyama, T. Watanabe, T. Hirata, and Nishiyama, “Wavelength modulation over 500 kHz of micromechanically tunable InP-based VCSELs with Si-MEMS technology”, IEEE J., Selected Topics in Quantum Electronics, vol. 15, pp. 528-534, May/June 2009, incorporated herein by reference. VCSEL's with fixed lasing wavelengths of 1310 nm and 1550 nm, utilized in the prior art, are described in N. Nishiyama, C. Caneau, B. Hall, G. Guryanov, M. H. Hu, X. S. Liu, M. -J. Li, R. Bhat, and C. E. Zah, “Long-wavelength vertical-cavity surface-emitting lasers on InP with lattice matched AlGalnAs-InP DBR grown by MOCVD”, IEEE J., Selected Topics in Quantum Electronics, vol. 11, pp. 990-998, September/October 2005, incorporated herein by reference.
In the prior art configuration of FIG. 1, a tuning range of 55 nm at a center wavelength around 1550 nm has been shown. The maximum tuning range is limited by the reflectivity bandwidth of the bottom DBR, which is determined by the ratio of the refractive indices of high-index and low-index materials. The reflectivity bandwidths of a DBR composed of alternating layers of AlGalnAs (high-index material) and InP (low-index material) are approximately 50 nm and 70 nm for center wavelengths of 1310 nm and 1550 nm, respectively. However, SS-OCT requires over 100 nm tuning range. Therefore, the VCSEL employing a DBR composed of AlGalnAs and InP and an active layer comprising quantum wells is not suitable for OCT applications.
To overcome this tuning range limitation, a tunable VCSEL with MEMS has been suggested, that employs a bottom mirror consisting of a DBR composed of alternating layers of AlGaAs (high-index material) and AlxOy (low-index material) that has a reflectivity bandwidth over 200 nm centered near 1300 nm. This type of tunable VCSEL has achieved a tuning range over 100 nm by optical pumping. The details are described in V. Jayaraman, J. Jiang, H. Li, P. J. S. Heim, G. D. Cole, B. Potsaid, J. G. Fujimoto, and A. Cable, “OCT imaging up to 760 kHz axial scan rate using single-mode 1310 nm MEMS-tunable VCSEL with >100 nm tuning range”, CLEO: 2011—Laser Science to Photonic Applications, PDPB2, 2011, incorporated herein by reference. In this approach, the active region comprises InP based multiple quantum wells (MQWs) epitaxially grown on an InP substrate. The bottom DBR is epitaxially grown on a GaAs substrate. Therefore, the materials in the active region and the DBR part cannot be grown on a single type substrate. The two wafers must be bonded together, and then the InP substrate needs to be removed in order to form the half VCSEL part. Bonding the GaAs and InP wafers and the removing the InP wafer requires a very complicated process and introduces potential reliability issues.
Quantum dot (QD) lasers have been investigated with the aim of replacing conventional quantum-well lasers. QD lasers have unique characteristics such as ultra-low threshold currents and low temperature sensitivity due to the three-dimensional quantum size effect. Quantum dot technology has progressed significantly by the self-assembling growth technique of InAs QD's on large GaAs substrates. Application of QD's to conventional edge emitting lasers (as opposed to VCSEL systems) has been accomplished by replacing quantum wells of the active layer by QD's. The high performance of 1.3 μm QD Distributed Feedback (DFB) lasers has been reported recently. These lasers are fabricated by molecular beam epitaxy (MBE) of 8 stacks of a high density QD layer with p-doped GaAs layers on a p-type GaAs substrate. The gain spectrum has been measured: a maximum net modal gain as high as 42 cm−1 at around 1280 nm is obtained, and the 3 dB gain bandwidth is approximately 65 nm. The details are described in K. Takada, Y. Tanaka, T. Matsumoto, M. Ekawa, H. Z. Song, Y. Nakata, M. Yamaguchi, K. Nishi, T. Yamamoto, M. Sugawara, and Y. Arakawa, “10.3 Gb/s operation over a wide temperature range in 1.3 μm quantum-dot DFB lasers with high modal gain”, Optical Fiber Communication Conference\National Fiber Optic Engineers Conference, (2010), Technical Digest, incorporated herein by reference.
A 1.3 μm VCSEL comprising QD's for fixed wavelength applications has also been reported recently: On a GaAs substrate, a bottom DBR composed of 33.5 pairs of n+-doped AlGaAs layer and n+-doped GaAs layer, an undoped active region composed of InAs/InGaAs QD's, a p-doped AlGaAs oxidation layer, and a upper DBR composed of 22 pairs of p+-doped AlGaAs layers and p+-doped GaAs layers, are grown by MBE. The lasing wavelength is around 1279 nm at room temperature. A small linewidth enhancement factor of 0.48 has also been reported, which can provide a narrow linewidth that is critical for OCT applications. The details are described in P. -C. Peng, G. Lin, H. -C. Kuo, C. E. Yeh, J. -N. Liu, C. -T. Lin, J. Chen, S. Chi, J. Y. Chi, S. -C. Wang, “Dynamic characteristics and linewidth enhancement factor of quantum-dot vertical-cavity surface-emitting lasers”, IEEE J. Selected Topics in Quantum Electronics, vol. 15, pp. 844-849, May/June 2009, incorporated herein by reference.
The discussion of the background herein is included to explain the context of the technology. This is not to be taken as an admission that any of the material referred to was published, known, or part of the common general knowledge as at the priority date of any of the claims found appended hereto.
Throughout the description and claims of the specification the word “comprise” and variations thereof, such as “comprising” and “comprises”, is not intended to exclude other additives, components, integers or steps.